As Kyle said. Basically IMO, you should swap to a V8 suspension for the 5 lug set up if for no other reason then improved braking. Hold out for front discs while you're at it. You should be able to find a complete 8" rear axle set up relatively cheap since a lot of people want to swap them out for a 9".
Easiest method is to find a COMPLETE 65/66 V8 based parts car, and strip the complete rear end with brakes, complete steering, including pitman arm, all steering parts, spindles, hubs, etc.
Now, considering getting a parts car that has GOOD parts is virtually impossible, you will be spending a bundle on replacing tie rods, brake parts, etc.
A complete drum to drum rear end, 8", common gears, for a 65/66 (unique width) runs about $100, plus shipping. Figure on new drums, and brake rebuild, and seals. Alternative is to find a Lincoln Versailles rear end, disk brake, and do the same work.
You will either need a new drive shaft, or modify your driveshaft to fit a 6 cyl output with a V8 rear end.
Front end, the relatively cheap method to get disk brakes is to use the 5 bolt granada ( or merc monarch) set up. Can be found, if you have a good day, in the junk yard, for $25, more likely, $150, will need new parts (pads, calipers, rotors, etc) but at least it is Autozone prices.
You then need all new tie rods, sleeves, pitman arm, etc. V8 pitman arm is not repo'd, and goes for around $35 used. Tie rods, sleeves, can be bought at AutoZone or other places.
I found a GOOD parts car for my '65 for $500. Plan on $300 in front end parts, $100 for Granada spindles, $25 for brake hoses, $35 fot the '75 Maverick dual master cylinder, $15 to turn the Granada rotors, $75 to modify driveshaft and $250 for an 8" rearend that will need new wheel cylinders, pads and drums turned at minimum. Well worth it though. Consider doing the Shelby drop while you're at it.
I used Granada Disk brakes up front. You need the Spindles,calipers,rotors from the granada.. Then You can get the Quick sterring kit for the pitman arm and idler arm. Then you need an Idler arm bracket, centerlink inner tierod ends, adjusting sleves, and granada outer tierod ends. I think its best to go through the brakeline books and find the perfect length and ends or lines rather than get the granada brake hoses and make an adaptor line to get them to meet the mustangs lines. Then a dual MC and proportioning valve for front disk rear drum setup.. I used a 74 maverick MC. This is a good time to rebuild the entire front end. Then you need a complete 8 or 9in. rear end. Many people have said we need new driveshafts, but my driveshafts from my V8 coupe and my 6cyl were the smae length, had the same yoke, and ujoint cap diameter. The only difference was my V8 shaft used ujoints with the locks on the inner portion of the cap vs the 6cyl used the clips on the outer end of the caps. Thats not to say The PO didn't really screw something up before I got ahold of one of them, but I did swap them once when I needed to replace my 6cyl.'s ujoints and needed to drive the car. I'm not saying its right and will work for you.